Multiple sclerosis and its animal model, experimental autoimmune encephalomyelitis (EAE), are a heterogeneous group of autoimmune, inflammatory diseases of the brain. The long-term goal of the proposed studies is to develop improved animal models to understand and treat disease subtypes, particularly those that are severe. The specific objective is to assess the role of CD8 T lymphocytes in mediating severity of EAE disease phenotype. The hypothesis is that, following initiation of central nervous system (CNS) inflammation by auto-reactive Th1 CD4 T lymphocytes, the subsequent parenchymal recruitment of CD8 T lymphocytes during a specific critical period leads to more severe clinical disease. To test this hypothesis, an adoptive transfer model of EAE was developed in (C57BL6xB10.PL) F1 mice. Preliminary data demonstrate that adoptive transfer of myelin basic protein (MBP)-specific (Acl-11) Th1 CD4 T lymphocytes can induce severe EAE in immune competent (C57BL6xB10.PL) F1 mice as compared to B10.PL mice. Histologic characterization, gene expression analysis, and CD8 depletion reveal a pathogenic role for CD8 cytotoxic T lymphocytes (CTL) in (C57BL6xB10.PL) F1 mice. These results suggest that, dependent on genetic background, CD8 T lymphocytes can mediate disease severity. Our hypothesis will be tested further in three Specific Aims. In Aim 1, we plan to use CD8-deficient and CTL effector-deficient mice to clarify the pathogenic roles of CD4 and CD8 T lymphocytes in acute lesions. Aim 2 focuses on defining the critical period of entry of CD8 T lymphocytes and associated CNS microenvironment changes in the early development of severe EAE. In Aim 3, we propose to isolate CNS-specific CD8 T lymphocyte clones, characterize them in vitro, and assess their ability to induce CNS inflammatory disease. These studies will provide a better understanding of CTL-mediated injury in inflammatory, demyelinating diseases of the CNS. In addition, this EAE model has clinical relevance because it more accurately reflects the complex roles of autoreactive CD4 and CD8 T lymphocytes in the pathogenesis of acute demyelinating lesions in multiple sclerosis. Since CTL's may initiate axonal injury in acute MS lesions with resultant chronic, progressive clinical deficits, this model should also be useful in screening potential MS therapeutics that inhibit CD4 T lymphocyte-initiated inflammation as well as CTL effector mechanisms. [unreadable] [unreadable]